Cosmological forecasts on thermal axions, relic neutrinos and light elements

TL;DR

This paper investigates how future cosmological observations can improve constraints on thermal axions, relic neutrinos, and light element abundances, enhancing our understanding of dark radiation and particle physics in the early universe.

Contribution

It provides forecasted constraints on axion and neutrino masses using simulated future CMB and galaxy survey data, including effects on Big Bang Nucleosynthesis.

Findings

Future observations can constrain axion masses to below 0.92 eV.

Sum of neutrino masses can be limited to under 0.12 eV.

Enhanced sensitivity supports multi-messenger analysis of early universe particles.

Abstract

One of the targets of future Cosmic Microwave Background and Baryon Acoustic Oscillation measurements is to improve the current accuracy in the neutrino sector and reach a much better sensitivity on extra dark radiation in the Early Universe. In this paper we study how these improvements can be translated into constraining power for well motivated extensions of the Standard Model of elementary particles that involve axions thermalized before the quantum chromodynamics (QCD) phase transition by scatterings with gluons. Assuming a fiducial $\Lambda$CDM cosmological model, we simulate future data for Stage-IV CMB-like and Dark Energy Spectroscopic Instrument (DESI)-like surveys and analyze a mixed scenario of axion and neutrino hot dark matter. We further account also for the effects of these QCD axions on the light element abundances predicted by Big Bang Nucleosynthesis. The most constraining forecasted limits on the hot relic masses are $m_{\rm a} \lesssim 0.92$ eV and $\sum m_\nu\lesssim 0.12$ eV at 95 per cent Confidence Level, showing that future cosmic observations can substantially improve the current bounds, supporting multi-messenger analyses of axion, neutrino and primordial light element properties.